An efficient and feature rich BitTorrent client for Windows sporting a very small footprint. It was designed to use as little cpu, memory and space as possible while offering all the functionality expected from advanced clients.

Typical memory use less than 4 MB
Incredibly small: 94 KB
Multiple simultaneous downloads
Smart bandwidth usage
File level priorities
Configurable bandwidth scheduling
Global and per-torrent speed limiting
Quickly resumes interrupted transfers
UPnP support (WinXP only)
Supports popular protocol extensions
Localized to different languages

http://www.utorrent.com